The study is based on the epidemic of 50,000 cases of viral hepatitis in the United States Army in 1942, traced to yellow fever vaccine prepared by the Rockefeller Foundation and contamniated with a virus of hepatitis, now shown to have been the hepatitis B virus (HBV). A serologic survey to identify the virus with certainty has been completed on 597 men--about 200 who suffered from acute hepatitis during the 1942 epidemic (Group I), 200 who received vaccine from one of the seven contaminated lots but were not clinically ill (Group II), and 200 who did not receive the Rockefeller vaccine (Group III). Two epidemiologic studies are being performed: 1) a mortality study of 55,000 men divided into three cohorts of approximately equal size, each defined as in the serologic survey, with primary liver cancer the chief end-point; and 2) a case-control study of 2,800 WWII Army Veterans discharged from Veterans Administration hospitals for liver cancer and 2,800 matched controls, the comparison to be based primarily on immunization history with attention to the lot number of the yellow fever vaccine. In the serologic survey, testing for anti-HBs and anti-HBc has identified the B virus as the source of the infection. In addition, anti-HB levels are high, and only one carrier (HBsAg+) was identified in Group I, none in Group II or III. The mortality study reveals no excess mortality from cirrhosis among either of the two groups infected with the B-virus, and at most a small excess of liver cancer, nothing like that expected from the Asian studies of carriers. The case-control study is still in the process but will be finished during the coming year. A report on the serologic survey was published in the New England Journal of Medicine and the paper on the cohort mortality study is in the final stages of preparation.